The production of elastomeric materials is known in the art from sources such as Kirk-Othmer Encyclopedia of Chemical Technology (4th edition, Volume 10, Pages 624-638, John Wiley & Sons, Inc., New York, 1993). Many elastomeric materials contain a urethane bond that is prepared by reacting a hydroxy-terminated polyether or polyester with a ployisocyanate at a molar ratio of about 1:1.4 to 1:2.5 (polyol to polyisocyanate). This is typically followed by reaction of the resulting isocyanate-terminated prepolymer with a polyamine to produce a high molecular weight urea/urethane polymer. Small amounts of monofunctional amines may also be included to control polymer molecular weight. Mechanical properties in the final polymer may be modified by the choice of the polyether or polyester glycol, the diisocyanate, the polyamine, and the monoamine used; and they can also be modified by the choice of the polyol-diisocyanate molar ratio.
Long-chain elastomeric urethane polymer molecules are substantially linear block copolymers containing relatively long blocks in which molecular interactions are weak, interconnected by shorter blocks in which interactions are strong. The weakly interacting blocks, commonly referred to as soft segments, typically derive from the polyether or polyester glycol component whereas the blocks having strong interactions derive from polyisocyanate and chain extender reactions, and are referred to as hard segments. The chain extension reaction is usually a coupling reaction between an isocyanate terminus and an amino group of a polyamine, resulting in a urea linkage. Thus, the resulting polymer of the combined hard- and soft-segments typically results in a poly(urea/urethane).
Polymers as described above have been used to prepare aqueous urethane dispersions. A urethane dispersion my be prepared, for example, by chain extending the reaction product of an organic diisocyanate or polyisocyanate and an organic compound having two or more active hydrogen atoms (in either a hydroxy or amino terminus), often using a small amount of an organic solvent. As the diisocyanate is used in stoichiometric excess, the reaction product, which may be a urea/urethane prepolymer is isocyanate terminated. Examples of prepolymer preparations such as these are described in U.S. Pat. Nos. 3,178,310, 3,919,173, 4,442,259, 4,444,976, and 4,742,095, among others.
Urethane dispersions are reported as being useful for preparing such diverse materials as coatings and bonds (U.S. Pat. No. 4,292,226); flexible solvent barriers (U.S. Pat. No. 4,431,763); adhesives (U.S. Pat. No. 4,433,095); and films (U.S. Pat. No. 4,501,852). Film applications include gloves, organ bags, condoms, ostomy bags, and the like. Conventional urethane dispersions have sometimes been found, however, to have insufficient physical or handling properties to make them a preferred material for such applications. Also, the use in the dispersion of certain relatively high-boiling solvents, such as N-methyl-2-pyrrolidone, can have adverse effects for some of these applications.
Despite the widespread use of aromatic polyisocyanates such as toluene diisocyanate (TDI), methylene diphenyidiisocyanate (MDI) and polymethylene polyphenylisocyanate (PMDI), an aliphatic polyisocyanate has sometimes been preferred for use in preparing an aqueous urethane dispersion. The aliphatic isocyanates, such as those disclosed in U.S. Pat. No. 5,494,960, are thought to have much higher stability toward hydrolysis while the prepolymer is dispersed in water. It is generally believed that, in such situation, a chain-extending reaction between an isocyanate and a polyamine takes place in a more controlled and predictable manner.
The reaction of a diisocyanate with a polyamine in water is, however, thought to be diffusion controlled, and it thus cannot be assured that all added polyamine is consumed during the reaction. Any unreacted polyamine remains with the polymer when it is fabricated from the disperion into a final product. When that product is intended for use in which it contacts human skin, the presence of such unreacted polyamines [such as ethylene diamine and other diamines as described in B. K. Kim, Colloid. Polym. Sci., 274:599-611, 1996 (“Kim”, which is incorporated in its entirety as a part hereof for all purposes)] may cause skin irritation or sensitivity for the user of the product. The presence of unreacted polyamines in a urethane dispersion can also cause an unpleasant odor in any product that is fabricated from the dispersion.
Films prepared from natural rubber latex are common, and are considered to have properties that are desirable from the perspective of comfort and utility. Unfortunately, natural rubber latex also includes proteins and other materials, such as sulfur containing curing agents, that can be irritating to the skin and may cause severe allergic reactions in some people.
Elastic films with good moisture management can provide protection from the environment, such as from germs and chemicals. Particularly with the increased potential threat from chemical and biological agents, the need of such materials is ever increasing. Recent incidents have shown need for comfortable gloves that can be worn by law enforcement and postal workers for a long period of time. Latex gloves usually have low puncture resistance, and moreover may pose additional health risks, including fatal allergic reactions by certain individuals. Nitrile gloves have good puncture resistance, but high modulus, so they can cause fatigue with long term use. Polyurethane elastomers may offer an alternative material choice, but some polyurethane gloves are found to get weak when exposed to water or rubbing alcohol. This would hinder the long term use of such gloves.
A need thus remains for urethane polymers that can be readily formed into, and fabricated from, a dispersion, and that, in the form of a fabricated article, have a reduced possibility of displaying characteristics that a user would find harmful or objectionable. Applicant has found that reducing or eliminating the content of unreacted polyamine in a urethane polymer results in the preparation of a urethane polymer that is useful for fabrication from a dispersion and yet has no tendency to cause skin irritation or to exude an unpleasant odor. Applicant has consequently proposed to reduce, and preferably eliminate, the content of unreacted polyamine in a urethane polymer by preparing the polymer without the use of a polyamine chain extender. Films formed from such polymers have been found to exhibit useful barrier properties toward water and some common alcoholic solvents, such as isopropanol, as well as useful mechanical properties such as low modulus at 100% elongation.